I’m based on Draenor for Horde and Silvermoon for Alliance. I main Horde so it’s where I tend to spend most of my time.
When new expansions come out you have to be online in game in order to be able to take part in launch. Even then sometimes Blizzard has other ideas. Both Draenor and Silvermoon crashed for several hours before they could let us play on launch.
For the first month or so you can expect queues at peak times. We used to have to get raiders to log early to be on time for raiders. The AH and Guild bank were practically unusable. Those were fixed eventually and now we can use both but they can be sluggish at peak times still, but nothing like it was.
There are minor delays in raid chat and instance chat sometimes but I’m not sure they are server based. Even in pugs everyone seems to have the same issues and we’re from all sorts of servers. It takes a few seconds for things you type to appear.
Those are the down sides. The perks are, busy server, good economy, big pool to recruit from and large number of guilds.